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Two U.S. astronauts and a Russian cosmonaut denied Tuesday they precipitated an off-target landing by pressing the wrong button as it emerged their Russian Soyuz craft may accidentally have gone into docking mode.

Commander Kenneth Bowersox, Dr Donald Pettit and flight commander Nikolai Budarin made a much steeper than expected uncontrolled entry for their landing on the plains of Kazakhstan on Sunday, enduring the crushing weight of gravity loads that made it difficult for them to breathe.

Budarin said the Soyuz TMA-1 craft appeared to have switched on a mechanism for docking with the International Space Station (ISS) while heading back to Earth. The three astronauts were making the first flight back from space since the Columbia shuttle disaster in February.

"During the descent, the mechanism for docking with the ISS turned out to be switched on," the ITAR-TASS news agency quoted Budarin as saying.

At a press conference, both he and Bowersox denied that human error had forced the Soyuz to abort its automatic landing before entering the Earth's atmosphere and change to a ballistic descent. Russian space officials had said on Monday that a cosmonaut pressing the wrong button could have caused the descent capsule to land 440km off course in the Kazakh steppe.

"You can never say for sure you didn't make a mistake. However we didn't see any sign on board that we made any errors," Bowersox told reporters at the Star City astronaut training centre outside Moscow. "The important thing is to analyse the situation, find out the truth, and see why the mistake occurred, if there was one, so that it doesn't happen to other people."

Budarin for his part insisted that the crew had not "issued any commands that might have led to ballistic entry."

"It's for the specialists to figure out" the cause of the malfuction, he said.

The Russia Journal online edition quoted a cosmonaut, Talgat Musabayev, as saying that Russian space experts believe the problem was caused by software in the guidance computer that was installed in the Soyuz TMA-1 spaceship. It was the first time the new, modified spaceship had been used in re-entry.

The Russian cosmonaut described their immediate sense of relief at their successful landing. "We lay on the ground, breathing in the fresh air which we had missed very much. We were happy above all that we made it back to Earth," he said.

The three men, who spent more than five months in weightlessness aboard the ISS, were only able to crawl on the ground for their first hour and a half of their time back on Earth, he said. Because of the ballistic entry, the crew had a very rough landing, enduring more than eight times the force of earth's gravity.

"It was easier than I thought it was going to be," Bowersox said. "There's a lot of pressure on your chest and when you come back from space, just one-G makes you feel heavy. So it's hard to breathe and your tongue sort of slips back in your head and toward the back of your throat."

Pettit, who still appeared weak and unsteady on his feet, described a crushing feeling of weight during the descent. "For a moment it felt like I was Atlas and I had the weight of the whole world on my shoulders," he said.

"It takes a while to get your shore legs back after an expedition like this. I've had a little more trouble walking around than others," he noted, explaining this was because it was his first space flight and he was taller and thinner than the others.